First you must have Free MC Boot. If you have it you can use the game launcher called OPL.

Then if your games is less than 4GB so you can put it on a USB stick or hard drive in a folder called DVD (Folder named with upper case) . if the game is less than 700 MB you will have to put it in a folder named CD (Folder named with upper case) .

If your game is higher than 4GB, then you will have to download and use a program on your computer named USB Advance. IT will split your game into 1GB parts directly on your USB stick.

The problems with USB is that you must procced to a defrag of your USB stick once you installed your game (use Defraggler on your computer) and be sure that everything is defragmented.
The other problem is that PS2 has USB 1.1 plugs which are slow, and you will suffer of longer loading times...